Thanks for explaining, I've run into this on my phone in other contexts and I was starting to think my phone screen is having its last days. Turns out it's expected? Usually I run into this when the screen brightness is at the lowest setting.
(There are exceptions: one could dial up pixel values on a dark scene and dial down the backlight settings to save power. But that depends on the image content.)
On manipulating the backlight to display a dark scene more power-efficiently: my TV that does this, if there's a small region of constant color (e.g. a TV station's logo) its brightness wavers noticeably as the rest of the scene changes.